The present invention relates generally to monitoring and controlling the current in inductor coils in the form of electromagnet coils, solenoids, or electric motor windings, and more particularly to controlling print hammer coil current.
An example of an electromagnet coil application is impact printing. In an impact printer there are many character positions per line on a printed page. A separate electromagnetic coil is used to move a print hammer for each print position. To print a desired character in a given print position, the electromagnetic coil for that print position is actuated with a current pulse at the proper time relationship with the desired character's position on a rotating metal character band. The speed of the rotating character band can be over 200 inches per second. The actuating current pulse causes the physical movement of the print hammer which quickly pushes the paper into a moving ink ribbon and both against only the desired character on rotating character band. The tolerance on the actuating current pulse is very important to print quality because the flight time of the print hammer is indirectly proportional to the energy in that current pulse. The contact time between the paper and moving ribbon and character band must be very small to avoid a print smudge or paper tear.
For impact printer applications that require accurate actuating current pulse control, a current chopping technique is used typically with two methods of controlling and sensing the current in an electromagnetic print hammer coil.
The `Top Drive` method has a control switch (PNP transistor or PFET) connected to the positive bias power supply. The coil is connected in between the switch and a sense resistor which is connected to the power ground node. With this method, the voltage across the sense resistor is directly proportional to the current flowing in the coil. There are two main problems with the `Top Drive` control method. Since the control switch is connected to the positive bias power supply, high voltage, high power pre-drive components are needed to control the switch device. The second problem is that the voltage sensed on the sense resistor requires that the power ground node be part of the sense circuitry. The power ground node is electrically very noisy.
The `Bottom Drive` method has the coil connected to the positive bias power supply. The control switch (NPN transistor or NFET) is connected between the coil and a sense resistor which is connected to the power ground node. With this method, the voltage across the sense resistor is proportional to the current flowing in the coil only during the time that the switch device is `ON`. This is the main problem with this control method, during the `Off` chopping cycle (control switch off) there is no way of monitoring the current flowing in the coil. A `Fixed Off Time` is usually used, but the average current level cannot be set as accurately with this method because of the non-linearity of the coil's inductance. The power ground node must be part of the sensing circuitry with this control method also.
The electrical environment of a magnetic assembly is usually very noisy. The noise is mainly a result of the following circuit functions. The switching of large currents through the inductance of the power supply cables. The low impedance of the magnetic coil's clamp diode during reverse recovery. The actuating and chopping of many magnetic coils at the same time. The physical compactness of the package layout allows noise coupling between components. Despite the noisy environment of the assembly, very accurate current level sensing and control must be preformed.
It is an object of the present invention to provide a print hammer coil current control that is low cost and integrated circuit compatible.
It is another object of the present invention to provide a print hammer coil current control that constantly monitors the current in the print hammer coil.
It is still another object of the present invention to provide a print hammer coil current that reduces noise supplied to other circuits and has improved noise immunity.